Optical amplifiers provide the capability to directly amplify an optical signal without first converting the optical signal to an electrical signal. Optical amplifiers are useful as repeaters and preamplifiers in optical communication systems. A semiconductor optical amplifier is typically constructed as a modified laser diode. An optical cavity is fabricated in a substrate with facets which act as mirrors on opposite ends of the optical cavity. When the device is appropriately biased, the optical cavity has optical gain. In order to provide operation as an amplifier rather than a laser, antireflection coatings are applied to the facets, or the facets are oriented at an angle relative to the optical axis of the cavity. An input optical signal is injected through one of the facets, is amplified in the optical cavity and passes through the facet at the opposite end of the optical cavity. Optical amplifiers are described generally by N. A. Olsson in "Lightwave Systems With Optical Amplifiers", Journal of Lightwave Technology, Vol. 7, No. 7, July 1989, pages 1071-1082.
Uses of optical amplifiers for applications other than or in addition to amplification have been proposed. Phase modulation of optical signals with semiconductor optical amplifiers is disclosed by J. Mellis in "Direct Optical Phase Modulation in Semiconductor Laser Amplifier", Electronics Letters, May 1989, Vol. 25, No. 10, pages 679-680 and by G. Grosskopf et al in "Characteristics of Semiconductor Laser Optical Amplifier as Phase Modulator", Electronics Letters, August 1989, Vol. 25, No. 17, pages 1188-1189. A modulation signal is combined with the bias current to effect phase modulation of the optical signal. Signal monitoring and control applications of semiconductor optical amplifiers are disclosed by M. Ikeda in "Signal Monitoring Characteristics for Laser Diode Optical Switches", Journal of Lightwave Technology, Vol. LT 3, No. 4, August 1985, pages 909-913; D. J. Malyon et al in "Laser Amplifier Control in 280 Mbit/s Optical Transmission System", Electronics Letters, February 1989, Vol. 25, No. 3, pages 235-236 and M. Gustavsson et al "Traveling Wave Semiconductor Laser Amplifier Detectors", Journal of Lightwave Technology, Vol. 8, pages 610-617, April 1990. In signal monitoring and control applications of optical amplifiers, a modulated optical signal injected into the optical cavity causes a change in the diode voltage, which can be measured at the current input terminals. Applications of semiconductor optical amplifiers as optical mixers, frequency converters and electrooptic mixers have also been proposed.
In all of these applications, the electrical response and the 3 dB electrical bandwidth of the optical amplifier are determined by the ability of the carrier density population N to respond to either an optical signal injected into the amplifier or an electrical signal applied to the amplifier current input terminals. In traveling wave optical amplifiers, the frequency response of the device is determined by the ability of the injected carrier population N to respond to the modulating signal. For a device operated well below saturation and for a typical carrier lifetime of 0.3 nanoseconds, the bandwidth is about 500 MHz. For a device operated at saturation, the bandwidth is increased to 1 GHz. For traveling wave optical amplifiers used as modulators, photodetectors, mixers or frequency converters, restriction of the bandwidth to the range of 500 MHz to 1 GHz is a serious limitation.
It is a general object of the present invention to provide improved optical communication systems.
It is another object of the present invention to provide improved semiconductor optical amplifiers.
It is a further object of the present invention to provide semiconductor optical amplifiers having a very wide electrical bandwidth.
It is yet another object of the present invention to provide a semiconductor optical amplifier capable of operation as a wide bandwidth optical phase modulator or optical signal monitor.